Fluorocarbon polymers in the category of thermoplastic halocarbon polymers generally have superior resistance to solvents and absorb no water at all. In addition, fluorocarbon polymers have the highest degree of resistance to weathering, heat and abrasion and exhibit the lowest degree of stickiness of all plastics.
Polytetrafluoroethylene (PTFE) is a representative fluorocarbon polymer. Because of its high resistance to heat and solvents, PTFE is used as a protective coat to prevent corrosion on the inner surfaces of pipes and as packings in chemical apparatus. PTFE also has low dielectric constant and dielectric loss while exhibiting high electrical resistivity so it is used as an insulator on electric cables. In addition, PTFE has high surface lubricity and is employed in making bearings and sliding members that require no lubrication. However, PTFE has a high melting point and cannot be easily molded by melt-forming techniques; therefore, PTFE must usually be formed by sintering procedures. With a view to improving the moldability of PTFE, copolymers of tetrafluoroethylene with other unsaturated monomers or fluorine-containing unsaturated monomers other than tetrafluoroethylene have been proposed. Furthermore, non-tetrafluoroethylene series fluorocarbon polymers which are thermoplastic and yet exhibit properties similar to those of PTFE have been developed.
Despite their superior properties, PTFE and the improved fluorocarbon polymers are expensive and their use will not be expanded unless they are laminated with inexpensive substrates. The problem, however, is that fluorocarbon polymers are non-adhesive and involve considerable difficulty in terms of being bonded to substrates. A variety of techniques have been studied in order to provide fluorocarbon polymers with improved adhesion to substrates. One approach is to modify the surfaces of fluorocarbon polymers by either the wet process wherein the surfaces of molded fluorocarbon polymers are treated with solution of alkali metals or by dry processes such as corona discharge, plasma discharge and sputter etching. According to other approaches, the surface of a fluorocarbon polymer is eroded by a special solvent that dissolves the polymer and the so treated fluorocarbon polymer is bonded to substrates; alternatively, the fluorocarbon polymer is physically bonded to substrates by sandwiching a glass mat.
Adhesive polymers which can be used as substrates to be bonded to fluorocarbon polymers have also been proposed. Among the polymers known as such adhesive substrates are certain ethylene-ethyl acrylate copolymers, ethylene-vinyl acetate copolymers and modified products thereof [see Japanese Patent Application Laid-Open Nos. 86748/81 and 12645/82]; epoxy-containing polyolefins [Japanese Patent Application Laid-Open Nos. 8155/82 and 212055/82]; and compositions comprising methyl methacrylate polymers grafted with vinylidene fluoride and methyl methacrylate series polymers [Japanese Patent Application Laid-Open No. 12646/82].
Chlorocarbon polymers as thermoplastic halocarbon polymers are generally flame-retardant, have superior resistance to water, acids and alkalis, are stable in many organic solvents and exhibit superior barrier properties against various gases and water vapor. Polyvinyl chloride (PVC) and polyvinylidene chloride (PVDC) are two representative chlorocarbon polymers.
PVDC has a softening point which is very close to the heat decomposition point and the plasticizers and stabilizers that can be used with PVDC are limited. Therefore, in order to provide PVDC having improved moldability, vinylidene chloride is copolymerized with such monomers as vinyl chloride, acrylonitrile and acrylate esters. PVDC is currently used in textile products such as fishing nets, insect-screen nets and tents, as well as in packing films, and coating latices.
PVDC has superior gas barrier properties but it is expensive and does not exhibit satisfactory degrees of heat resistance and stiffness when used alone. Therefore, in the food packing industry, PVDC is used in the form of coatings of dispersions or, alternatively, it is laminated with substrates such as polyolefins by coextrusion molding. Illustrative uses of PVDC are as films and containers. In the case of containers, a dispersion of PVDC is difficult to apply and requires a drying step. In order to avoid this complexity of operations, it is desirable to fabricate PVDC containers by the simpler coextrusion molding rather than the coating of dispersions.
PVDC is not highly adhesive and attempts have been made to improve the adhesiveness of its dispersion to the substrates by introducing functional groups through copolymerization with other monomers. Adhesive polymers which are suitable for use as substrates that adhere to PVDC have been proposed and they include: ethylene-vinyl acetate copolymers [see Japanese Patent Application Laid-Open Nos. 66770/74 and 26876/78; and Japanese Utility Model Application Laid-Open No. 83339/78]; thermoplastic polyurethanes [Japanese Patent Application Laid-Open Nos. 106584/79 and 217151/85]; and certain polyesters [Japanese Patent Application Laid-Open No. 152085/79].
The thermoplastic halocarbon polymers proposed by the aforementioned prior art techniques are not completely satisfactory in terms of moldability, workability and adhesive strength and the types of substrates to which they can be bonded are limited. For example, Japanese Patent Application Laid-Open Nos. 86748/81 and 12645/82 disclose laminates of thermoplastic fluorocarbon polymers and ethylene-vinyl acetate copolymers; if, with a view to providing improved peeling strength, a large amount of vinyl acetate is introduced in the form of a copolymer with ethylene, the resulting copolymer has lowered mechanical strength and yet its bonding strength to the fluorocarbon polymer is not significantly improved. As for chlorocarbon polymers, they are in most cases laminated by coextrusion molding with ethylene-vinyl acetate copolymers as proposed in Japanese Patent Application Laid-Open Nos. 66770/74 and 26876/78 and in Japanese Utility Model Application Laid-Open No. 8339/78, wherein the ethylene-vinyl acetate copolymer is laminated with PVDC. Not only does the PVDC adhere poorly to polypropylene and high-density polyethylene but it also has such a low degree of heat resistance that it is unsuitable for use as a food packing material which must withstand high temperature sterilization as in boiling or retorting.